A memory cell of the EEPROM type generally comprises a transistor having a floating gate allowing the storage of the item of data, a control gate, a source region and a drain region. Such a memory cell uses the principle of the non-volatile storage of charges on the floating gate of the transistor. Conventionally, the writing of an item of data comprises an erase cycle followed by a programming cycle.
The programming is carried out by the Fowler-Nordheim effect and comprises the injection by tunnel effect of electrons from the floating gate towards the drain whilst the erasing, which is also carried out by the Fowler-Nordheim effect, comprises an injection by tunnel effect of electrons from the drain towards the floating gate.
Conventionally, the programming of a memory cell of the EEPROM type is carried out by connecting the control gate to ground and by applying a relatively high programming voltage to the drain, typically 13 Volt. Moreover, with regard to the source, there are two possibilities: either the source is left floating or it is pre-charged at a non-zero pre-charge voltage.
In the first case (source left floating), the potential of the source rises during the programming, the floating gate is charged positively and the memory cell goes, at the end of the programming, into a conducting state but does not conduct because, as the source is floating, it presents infinite impedance.
In the second case (pre-charging the source), the floating gate is also charged positively during the programming but the cell does not go into a conducting state because the pre-charge voltage of the source is chosen such that the difference between the potential of the floating gate and the potential of the source always remains below the threshold voltage of the transistor of the memory cell.
Thus, in all cases, the memory cell does not go into conduction during the programming.
Moreover, the write cycle (erasure and then programming) is of fixed duration, typically 4 ms, which can result in over-programmings, that is to say in quantities of charge uselessly stored on the floating gate that are too large, which has a negative impact on the lifetime of the memory cell, as well as in a dispersion of the conduction thresholds of different programmed memory cells.